1. Field of the Invention
The present invention relates to an electro-modulating device for modulating light from a light source, in particular to an electro-modulating device for use as part of an opto-electronic communication network.
2. Discussion of the Background Art
In an opto-electronic communication network, it can be desirable to modulate light by passing the light through a modulator. Normally the modulator is formed from a modulator material whose optical properties depend on the electric field applied across it, so that modulating the electric field across the modulator material results in a modulation in the intensity and/or phase of light passing through it.
It is known to modulate light with a modulator having a light input and a light output. One optic fibre is coupled to the modulator input so that light of constant intensity can be passed into the modulator, and another optic fibre is coupled to the modulator output so that modulated light leaving the modulator can be received. This requires two fibres to be coupled to the modulator.
It is an object of the present invention to address the above issues.
According to the present invention, there is provided an electro-modulating device comprising a modulating element, the modulating element having a modulating medium for modulating light passing therethrough, an optical input-output surface by which light both enters the medium prior to modulation of the light and exits the medium after modulation of the light, a light reflector, and electrodes for applying an electric field across the modulating medium, wherein:
the input-output surface, the medium and the reflector are arranged so that light enters the medium through the input-output surface, travels through the medium towards the reflector, is reflected by the reflector to travel back through the medium towards the input-output surface, and exits the medium through the input-output surface;
the electric field is transverse to the direction of propagation of the light traversing the medium between the input-output surface and the reflector; and
the refractive index of the medium is responsive to the applied electric field so that the intensity and/or phase of the light exiting the input-output surface is dependent on the applied electric field.
It will be appreciated that the light may be visible but may alternatively be invisible electromagnetic radiation, such as infra red radiation.
The term refractive index is intended to include the real part and/or the imaginary part thereof, such that a change in the refractive index of the modulating medium can result in a change in the phase and/or intensity of light passing therethrough.
Only a single optic fibre needs to be coupled to the electro-modulating device, since the input-output surface serves as both an input and an output.
Because the electric field is transverse to the direction of light traversing the modulating medium, it will be easier to position the electrodes such that they are less likely to cause an obstruction to the entry and exit of light into and out of the modulating medium.
The effective path length of light travelling through the modulating medium will be approximately twice the distance between the optical input-output surface and the reflector, thereby increasing the magnitude of the modulation achieved with the electro-modulating device, or reducing the voltage that needs to be applied to the electro-modulating device to a achieve a given degree of modulation.
Preferably, the modulating element will be formed from a section of semiconductor wafer and the modulating medium will be formed from an active layer on or in the semiconductor wafer, the active layer having a plurality of edges and the input-output surface residing on an edge of the active layer. If an optic fibre having an end portion is used to feed light into and out of the modulating medium, the end portion of the fibre can conveniently be positioned close to the input-output surface, in line with and parallel to the active layer forming the modulating medium, thereby making it easier to connect the electro-modulating device to the optic fibre.
If the modulating element is formed from a semiconductor wafer, the modulating medium may be an active layer situated between a first layer of conducting semiconductor and a second layer of conducting semiconductor, the first and second layers of conducting semiconductor forming the electrodes for applying an electric field across the active layer, normal to the active layer. Since the conducting layers of semiconductor can be very close to one another on either side of the modulating medium, the electric field applied across the modulating medium will be increased.
The modulating element may be a mesa with side walls etched on the semiconductor wafer, such that the edges and/or ends of the active layer lie on the side walls of the mesa. The mesa may be buried, or the modulating element may be formed by a ridge structure.
Alternatively, the edge of the active layer may reside on cleaved side walls formed by cleaving the wafer.
The electro-modulating device will preferably include a mounting surface on which there is mounted the modulating element. The mounting surface will preferably have a clamp, housing, adhesive area or other securing means for securing the end portion of an optic fibre such that light from the fibre can be coupled into and out of the modulating medium through the input-output surface. In a preferred embodiment, the mounting surface will be formed by a silicon substrate having a V-groove etched thereon for receiving the end portion of an optic fibre.
The mounting surface may have a light guide formed thereon for guiding light into and out of the modulating element. The modulating element may be integrally formed with the mounting surface, which will preferably be a semiconductor substrate. If the modulating element and the mounting surface are integrally formed, the modulating medium and the light guide will preferably be formed from a continuous layer of semiconductor.
The substrate may be formed from layers of silicon and silicon oxides and/or nitrides in which case the light guide may be defined on the substrate by etching.
The reflector may be formed by at least one layer of reflective material deposited on an end wall of the modulator element, or alternatively the reflector may be mounted on the mounting surface, facing an end wall of the modulator element. The reflector may be in contact with the active layer or there may be a gap between the active layer and the reflector. It will be appreciated that the reflector need not be entirely reflective and may allow some transmission therethrough.
The reflector may be a distributed bragg reflector having a plurality of layers, or the reflector may be a layer of metal, deposited on an end wall of the modulator element. A layer of insulator may be provided between the metal layer and the end wall of the modulator element to reduce the risk of the electrodes on the modulator element being short circuited by the metal layer.
The modulating medium will preferably be an electro-optic material, preferably semiconductor such as GaAs or InP, or a semiconductor containing In, Ga, As, and P or Al. If the electro-modulating device is intended to modulate the intensity of light, the modulating medium will preferably be a material whose absorption coefficient is dependent on the applied electric field. If the electro-modulating device is intended to modulate the phase of the light, the modulating medium will be a material where at least the real part of the refractive index changes with applied electric field. The modulating layer may have a multiple quantum well layer.
In a preferred embodiment, the modulating element is formed from a semiconductor wafer wherein the modulating medium is formed from a multiple quantum well layer of InGaAsP, and each electrode is formed from a layer of conducting InP. The modulating medium may be doped with one polarity of doping and the electrodes may be doped with an opposite polarity of doping in order to form a p-n junction between each electrode and the modulating medium.
The modulating element may be formed from a polymeric material, and may be coupled with a waveguide also formed from a polymeric material.